Thermosetting resin compositions, such as epoxy resins, are widely used for various adhesion materials, molding materials and the like. One example of a curing agent used for such a thermosetting resin composition is a latent imidazole-based curing agent. Since this latent imidazole-based curing agent does not exhibit a curing performance in a normal storage state, it is widely used to turn thermosetting epoxy resin compositions into a one-component curing composition having good handleability and good storage stability. Representative examples of such a latent imidazole curing agent include microcapsule-type agents which have an epoxy resin cured matter coating the surface of imidazole compound particles which are capable of curing the epoxy resin.
However, such a microcapsule-type latent imidazole curing agent has a coating which is both mechanically and thermally relatively stable. Thus, to initiate the curing reaction, it is necessary to heat to 180° C. or more under pressure. Therefore, there has been the problem that such a microcapsule-type latent imidazole curing agent is not applied to the low-temperature curing type epoxy resin compositions of recent years.
Accordingly, latent curing agents have been proposed which exhibit low-temperature and rapid curability even without the use of a toxic promoter, such as antimony and the like. For example, Patent Document 1 describes a microcapsule-type aluminum chelate-based latent curing agent having a polyvinyl alcohol coating layer formed on the surface of aluminum chelating agent particles (mother particles) which can generate a protonic acid in conjunction with a co-catalyst silanol (silane coupling agent etc.) to cause a cyclic ether (epoxy compound or oxetane compound) to undergo cationic ring-opening polymerization. In Patent Document 1, the polyvinyl alcohol coating layer is formed on the surface of the mother particles by a hybridization method in which polyvinyl alcohol microparticles (daughter particles) having a hydroxyl group, which reacts with the aluminum chelating agent, fuse and stick to the surface of the mother particles. Furthermore, Patent Document 2 describes a microcapsule-type aluminum chelate-based latent curing agent having a coating layer formed on the surface of mother particles. In Patent Document 2, fluorine resin microparticles (daughter particles) which do not have a functional group that can react with the aluminum chelating agent are made to stick to the surface of the mother particles by static electricity, and then the fluorine resin microparticles are fused by a hybridization method to form an integrated surface.
It is noted that details of the curing step for an aluminum chelate-based latent curing agent are described in paragraphs 0007 to 0010 of the above-mentioned Patent Document 1.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-368047    [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-363255